Various types of optical systems utilize reflective objectives. These systems include for example, telescopes, UV microscopes, measurement systems operating in UV spectral range, etc.
The commonly used reflective objective is the Schwarzschild-type objective. As illustrated in FIG. 1, this objective includes a first apertured spherical mirror M1 and a second spherical mirror M2 accommodated in a spaced-apart relationship along an axis of symmetry of the mirrors. The second mirror M2 is typically mounted on a so-called spider (shown schematically), which complicates the construction. Moreover this configuration suffers from diffractive effects that might occur on the spider, obscuration caused by the mirror M2 and spider which results in about 20% aperture reduction, relatively high-angle reflection which causes polarization distortion, a high minimal numerical aperture (about 0.3) with significant angular obscuration near the optical axis.
Various imaging/measurement techniques require operation with a broad spectrum, i.e., from infra-red (IR) to deep ultraviolet (DUV). In this connection, refractive optics, due to limited availability of transparent materials at short wavelengths, cannot be used in a spectral range down to 190 nm. Moreover, refractive optics, due to unavoidable dispersion effects, causes high chromatic aberrations and thus does not enable illumination of a small spot needed for measuring in small measurement sites.